An imaging method and apparatus involving electrocoagulation of a primarily aqueous dispersion has been disclosed by the Castegnier et al. patents (e.g., U.S. Pat. Nos. 3,892,645, 4,555,320, 4,661,222, 4,895,629, 5,538,601, 5,609,802, 5,693,206, 5,727,462, 5,908,541 and 6,045,674) wherein an electric current is passed between a positive electrode (or an array of positive electrodes) and a negative electrode (in an array of negative electrodes) to produce an electrocoagulated deposit on the positive electrode. An imagewise electrocoagulated deposit may be transferred to a receiver such as paper to form a single color image, e.g., a black image, on the paper. Alternatively, imagewise electrocoagulated deposits of different colors may be sequentially deposited, e.g., on a positively biased belt, so as to form a full color image for subsequent transfer to a receiver. A squeegee blade apparatus for removing excess liquid is disclosed in the Castegnier et al. patents (U.S. Pat. Nos. 5,928,486 and 6,090,257). A difficulty inherent in the electrocoagulation technique is that image uniformity requires an extremely accurate distance between each pair of opposing positive and negative electrodes, typically about 50 micrometers. Moreover, the image resolution is limited by the diameter of individually addressable electrodes and also by the fact that these electrodes must be isolated from one another by a thickness of insulating material between them. There are other difficulties, e.g. that the electrical power density required for creating an electrocoagulated image is relatively high, that special materials are needed to suppress unwanted gas generation near the electrodes, and that electrodes must be protected against electrolytic erosion. The Castegnier et al. patent (U.S. Pat. No. 4,555,320) discloses a relatively low resolution of 200 dots per inch requiring 25 watts of power (50 volts, 500 ma) to produce 100,000 developed dots per second, which is equivalent to about 100 microcoulombs of charge delivered in about 0.4 second per developed dot, resulting in a significant power density of about 4.1 watts/in2 if every imaging pixel is developed (maximum density flat field image). The Castegnier patent (U.S. Pat. No. 4,764,264) discloses a resolution of 200 dots per inch requiring 25 watts of power to produce 1,000,000 developed dots per second, each developed dot requiring passage of 25 microcoulombs of charge.
In related copending U.S. patent application Ser. No. 09/973,244, now U.S. Pat. No. 6,682,189 entitled Ink Jet Imaging Via Coagulation On An Intermediate Member by John W. May, et al., the contents of which are incorporated herein by reference, certain embodiments are disclosed for using an ink jet device to form an ink image on an intermediate member, which ink is an electrocoagulable ink. By jetting a predetermined variable number of droplets on each imaging pixel of an operational surface of the intermediate member, the resulting ink image on the intermediate member has a predetermined variable amount of coagulable ink per pixel. The ink image is moved into contact with an electrocoagulation member, which electrocoagulation member makes physical contact with the variable amounts of liquid of the ink jet image on the intermediate member. Passage of electric current between an electrode included in the electrocoagulation member and a sub-surface electrode included in the intermediate member results in passage of corresponding currents through the variable amounts of electrocoagulable ink, thereby causing an imagewise formation of coagulate deposits on the intermediate member. An excess liquid phase not included in the coagulate deposits is removed from the coagulate deposits while the coagulate deposits remain on the intermediate member, and the coagulate deposits are subsequently transferred to a receiver member. There are certain limitations, which may be associated with the above-described embodiments. These limitations include: (1) a difficulty associated with providing a small enough gap, between the operational surface of the intermediate member and the electrocoagulation member, so that every differing amount of electrocoagulable ink in the ink image can be contacted by the electrocoagulation member, i.e., so that electrocoagulation can occur efficiently at every imaging pixel where there is ink; (2) if, in fact, the gap is made thus sufficiently small, there is a difficulty with a possible blurring of the image as a result of a squashing of the larger amounts of the variable amounts of ink; (3) after the coagulate deposits are formed on the intermediate member, there is a difficulty in efficiently removing the corresponding variable amounts of excess liquid phase from the coagulate deposits; (4) owing to a varying thickness from pixel to pixel of the coagulate deposits, a high efficiency of transfer to a receiver of the thinnest of such deposits may be difficult to achieve.
In related copending U.S. patent application Ser. No. 09/973,239, now U.S. Pat. No. 6,769,423 entitled Ink Jet Process Including Removal Of Excess Liquid From An Intermediate Member by Arun Chowdry, et al., the contents of which are incorporated herein by reference, certain embodiments are disclosed for using an ink jet device to form a colloidal ink image on an intermediate member, which ink is nonaqueous colloidal dispersion of electrically charged pigmented particles in an insulating carrier liquid, similar to a liquid developer for use in electrostatography. By jetting a predetermined variable number of droplets on each imaging pixel of an operational surface of the intermediate member, the resulting colloidal ink image on the intermediate member has a predetermined variable amount of colloidal dispersion per pixel. In one of the disclosed embodiments, the colloidal ink image is moved into proximity with an electrode member, which electrode member makes physical contact with the variable amounts of liquid of the ink jet image on the intermediate member. An electric field applied between an electrode included in the electrocoagulation member and a sub-surface electrode included in the intermediate member urges the charged particles of the dispersion to form a concentrated image on the operational surface of the intermediate member. An excess carrier liquid not included in the concentrated image is removed from the concentrated image while the particles remain on the intermediate member, and the particles thus left behind on the operational surface are subsequently transferred to a receiver member. In other disclosed embodiments, the electrode member does not touch the ink image, and in yet other disclosed embodiments, a corona charging device is used to charge the variable amounts of liquid in the ink image, thereby producing internal electric fields within the variable amounts of liquid for urging the corresponding charged particles in each imaging pixel to migrate to the operational surface. There are certain limitations, which maybe associated with one or more of the above-described embodiments. These limitations include: (1′) a difficulty associated with providing a small enough gap, between the operational surface of the intermediate member and a contacting electrode member, so that every differing amount of ink in the ink image can be contacted by the contacting electrode member, i.e., so that particle migration can occur efficiently at every imaging pixel where there is ink; (2′) if, in fact, the gap is made thus sufficiently small, there is a difficulty with a possible blurring of the image as a result of a squashing of the larger amounts of the variable amounts of ink; (3′) after the concentrated image is formed on the intermediate member, there is a difficulty in efficiently removing the corresponding variable amounts of excess carrier liquid; (4′) owing to a varying thickness from pixel to pixel of the deposits of migrated particles, a high efficiency of transfer to a receiver of the thinnest of such deposits may be difficult to achieve.